Modern commercial aircraft have a variety of high-lift devices, such as leading edge flaps, to increase the lifting ability, while maximizing the lift-to-drag ratio of the aircraft. Typically, high-lift devices may be actuated between a stowed position for high-speed aerodynamic performance and an extended position for low-speed aerodynamic performance. In the extended position, such devices are deployed to increase the camber of the wing and, therefore, increase the lift of the aircraft at slower speeds. However, the effectiveness of such leading edge high-lift devices is determined, at least in part, by airflow leakage at the ends of such devices, particularly adjacent to fixed structure along the wing span.
For example, in an airplane where an engine nacelle is closely coupled or mounted to the wing, such as a Boeing 737, the leading edge flap is contoured to clear the engine nacelle when the flap is deployed. As a result, the leading edge of the flap has a uniquely arcuate shape spaced from the nacelle sufficiently, such that airflow leakage is present between the flap and the nacelle.
In the past, leading edge flaps of such airplanes have included a pair of doors hingedly attached to the leading edge flap. One of the doors is attached adjacent to the flap and extends from the outboard facing side of the engine strut to the inboard facing edge of the leading edge flap. The second door is hingedly attached to the arcuate section of the leading edge flap. Each door has a lower edge that is curved in a manner that conforms with the circumference of the engine nacelle. The lower free edge of the leading edge flap is formed of a resilient material. The material engages and compresses against the nacelle when the minimum clearance of the flap actuation is reached. The material expands as the flap is extended into the extended position, such that the material is compressed from a greater-to-lesser extent during actuation of the flap.